1. Field of the Invention
The present invention relates to a method for transmitting Video on Demand (VoD), and more particularly, to a method for transmitting Near Video on Demand (NVoD) using Catch and Rest (CAR) and sub-channels which can reduce service latency for high-end terminals.
2. Description of the Related Art
With the rapid development of communication and network technologies in recent years, Video on Demand (VoD) has been considered as a promising service which enables the users to request and play the desired multimedia contents. There exist many ongoing researches focusing on efficient transmission of VoD.
The transmission methods for VoD can be classified into True VoD (TVoD) and Near VoD (NVoD).
With TVoD, the users can be serviced with the desired contents immediately upon request at anytime. However, since the users are serviced via unicast, it is not efficient in terms of bandwidth utilization when multiple users request the same content.
On the contrary, NVoD uses multicasting which is suitable for providing a same service to multiple users. Unlike TVoD, however, service latency is high which means that the users have to wait for some time period after they make a request. Moreover, unlike TVoD, NVoD does not provide VCR function. Despite these drawbacks, high bandwidth efficiency makes NVoD a good candidate for servicing highly popular contents to a large number of users.
The transmission methods employed for NVoD can be classified into three types: batching, patching and periodic broadcasting.
The batching scheme puts the user requests for the same video into a single “batch” which arrive close in time, and provides service to the users in the batch by sending the video via a single multicast channel. The patching scheme allows late arriving users to join an ongoing multicast channel and sends the missing portion of the video of the earlier part via unicast. With the periodic broadcasting scheme, a single VoD content is divided into segments which are periodically broadcast on different channels. Compared to the former two schemes, respective video streams are allocated to fixed channels. Therefore, it is effective for popular contents of many simultaneous requests.
The existing broadcasting schemes such as Fast Broadcasting, Pagoda Broadcasting, Harmonic Broadcasting effectively reduce the service latency, which refers to the time between the request for VoD and the playing of the contents. The schemes above assume that the terminal has unlimited bandwidth and buffer capacity. This is unrealistic in the wireless/wired convergence environment where terminals have different capabilities.
Accordingly, so as to effectively support heterogeneous terminals, new broadcasting schemes such as HeRo, BroadCatch and Catch and Rest (CAR) have been introduced. These scheduling schemes'allocate different bandwidth to the time slots of the segments, so that each terminal can choose the time slot to start receiving the VoD contents according to its capability.
Among these schemes, CAR allows small service latency and buffer space requirement especially for low-end terminals. However, use of replicated channels in CAR makes the segment size relatively large, which results in increased service latency for high-end terminals.